that became; who became이 is the subject particle used after a noun ending in a consonant, and 된 is the past adnominal form of 되다 (“to become”). Together, 이 된 describes someone or something that became what follows.
KO: 그는 세계 챔피언이 된 소감을 말했어요.
EN: He talked about how it felt to become world champion.